When asking a preview from .rst file, we walk back in the file tree
to search for a conf.py sphinx config file.
(the limits are ten level up or the project tree basedir)

the results of sphinx-build commands are keeped around
for each basedir in the tmp folder in gnome-builder-sphinx-build-XXXXXX
like folders (so we can update it after each key strokes or
triggering another preview with the same basedir)